"Agitate: Negotiating the Photographic Process" at SF Camerawork makes a neat postmodern addendum to the recent exhibition that Stanford's Cantor Center devoted to Eadweard Muybridge and the quest for true stop-motion photography.

"The agitated foliage, the running stream, the flying clouds, and the motions of living animals, all destroy the picture in which they occur," a novice wrote in the Edinburgh Review in 1843.

The line serves as an epigraph to "Agitate."

Behind the early camera workers' ambition to capture motion accurately as it happens -- instant by instant -- lay their faith in the indexical nature of photography: the direct causal link between subject and image.

The artists in "Agitate" play not primarily with speed but with indexicality, the paw prints of photographic "truth."

The camera had no part in many of the works shown.

To make the series "A Tree Is a Tree Is Not a Tree" (2001), Carlos Motta exposed color photographic paper in a literal but unconventional sense. He tacked sheets of it to forest trees and left them at the elements' mercy for a week or more.

Rain, light, temperature changes and the chemistry of tree bark produced the thin tracery of colored lines on the slick mauve ground of "Untitled (Tree No. 9)" (2001).

Motta's pieces bring dimly to mind the stain paintings of Morris Louis (1912-1962). But whereas Louis tried to marry aesthetics and meaning irrevocably, Motta severs them. He neither saw his works as they evolved nor can know precisely how they will change with time.

Yet his abstract pages are "of trees" with an abject fidelity no ordinary photograph can boast.

Binh Danh also relied on organic processes to make his chlorophyll prints: images literally printed on tree leaves.

For his current series, "Searching for the Cosmos," Danh downloaded images of the night sky from the Internet and converted them digitally to transparent negatives. He then overlaid the negatives on individual leaves, sandwiched between sheets of glass, and left them in the sun to "print." He kept the leaf stems dipped in water.

Where shadows on the negative deprived the leaves of light, the interrupted photosynthesis yielded an image like the starry heavens that span "Night Sky" (2003), a veined, almond-shaped leaf embedded in resin to "fix" it.

Beauty of concept and of outcome come into a rare, fine balance here.

Besides devising an almost Transcendentalist identification of vastness and natural detail, Danh brings back to life the ancient symbol of the world tree that in ancient myth connected all the realms of being.

Like Motta's work, Danh's brings John Cage to mind, both his declared intent to mimic "nature in her manner of operation" and his use of star chart overlays in generating the score of his "Etudes Australes."

Less lyrical but no less ingenious is Kate Farrall's "darkroom mapping" (2003), the only ensemble here in which images' content implies the prints' positions in the exhibition space.

Noticing how much light enters the theoretically lightless color darkroom in which she works, Farrall decided to map the room's faintly visible landmarks by exposing sheets of chromogenic paper directly to them.

The softly glowing light switch registered as a squarish blush of deep magenta on the paper's blazing white. It hangs about chest high at the entrance to the enclosure here that roughly duplicates her work space. The processor appears as a mirror-image keypad.

Another print at floor level records a breath of deep red shadow: light bleeding from under the door.

Altogether the prints suggest something like a reconstructed crime scene or some sophisticated variant of blind man's buff.

But most excitingly, like "Agitate" as a whole, they reclaim photographic processes and materials as a domain of thought made visible.